Conventionally, for colorants for inkjet recording ink, dyes have been used, however, dye inks have shortcomings in that they are poor in water resistance and weather resistance and easily bleeds on regular paper. To remedy these shortcomings, an ink using a pigment as a colorant is proposed.
For example, Patent Literature 1 proposes a method for alleviating color bleed using a self-dispersible carbon black, however, dyes are used for the color inks for the ink set demonstrated in the publication, and the color ink is inferior in water resistance and weather resistance. In addition, the color inks have a shortcoming in that they are inferior in feathering on regular paper to pigment inks. The method of alleviating color bleed proposed in the publication utilizes a quick separation of solid contents from a solvent, however, it is difficult to prevent color bleed on media which are poor in penetration.
Patent Literature 2 proposes an ink set containing inks having a difference in surface tension between the inks of 5 mN/m to 50 mN/m.
Patent Literature 3 proposes an ink set having a difference in viscosity between the inks of 0.7 mPa·s to 4 mPa·s. However, when there is a large difference in physical properties between inks, it is unfavorable because of difficulty of selecting members for ink nozzle and ink head peripherals and designing systems such as cleaning mechanism. Particularly when the ink viscosity differs between inks, the amount of discharged ink-drop varies at every discharge time, and the dot diameter differs between color inks to make it difficult to perform image processing. Thus, it is preferred that an ink set has smallest possible difference in physical properties between inks.
Patent Literature 4 proposes a method for reducing occurrences of color bleed using water-soluble organic solvents having a difference in solubility parameter of 4 or more between a black ink and color inks. However, with the ink compositions demonstrated in the present invention, namely, when an ink set using an ink using a water-insoluble color material was used to record information on poorly ink-absorbable coated paper, it was impossible to prevent color bleed even when water-soluble organic solvents having a difference in solubility parameter of 4 or more between color inks.
Patent Literature 5 proposes preventing color bleed by defining the initial contact angle of ink with regular paper, the difference in dynamic contact angle, and the rate of dot-dimension change. The initial wet property of ink relative to recording media effects color bleed, however, it is difficult to prevent color bleed by only controlling the wet property of ink relative to recording media.
Further, Patent Literature 6 defines the range of contact angle between a first liquid and a second liquid, however, similarly to Patent Literature 5, it is difficult to prevent color bleed by only controlling the wet property of ink relative to recording media. In addition, since the ink composition used in Patent Literature 6 has a large water content, it easily causes a phenomenon called cockling where the printed medium is deformed like wavy paper.
Accordingly, an ink which excels in image water resistance and image weather resistance and allows excellent image formation without substantially causing color bleed not only on regular paper but also on poorly water-absorbable printing paper has not been realized yet. Here, poorly water-absorbable paper means printing coated paper whose surface is applied with an inorganic pigment such as calcium carbonate and kaolin, and such paper is not generally recognized as paper having inkjet suitability because it has a low rate of ink absorption when printed with a water inkjet ink, and color bleed easily occurs.
Inkjet printing enables easy printing because of no need of plate-making, and water inks have less environmental burdens. For these reasons, it is desired to print images at a quality as high as those seen in commercial printing such as offset printing and gravure printing, using a water inkjet.
Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 2004-352996
Patent Literature 2 Japanese Patent Application Laid-Open (JP-A) No. 2005-154605
Patent Literature 3 Japanese Patent Application Laid-Open (JP-A) No. 2005-154607
Patent Literature 4 Japanese Patent Application Laid-Open (JP-A) No. 2002-275402
Patent Literature 5 Japanese Patent Application Laid-Open (JP-A) No. 2003-321631
Patent Literature 6 Japanese Patent Application Laid-Open (JP-A) No. 2004-338392